A known system for ejecting ink to form images on a moving web of media material is shown in FIG. 3. The system 10 includes a web unwinding unit 14, a paper conditioning unit 16, a media preparation station 18, a pre-heater roller 22, a plurality of marking stations 26, a turn roller 30, a leveling roller 34, and a spreader 38. In brief, the web unwinding unit 14 includes an actuator, such as an electrical motor, that rotates a web of media material in a direction that removes media material from the web. The media material is fed from the unwinding unit 14 through the paper conditioning unit 16 and the media preparation station 18 along a path formed by the pre-heater roller 22, turn roller 30, and leveling roller 34 and then through the spreader 38 to a rewinder 40. The paper conditioning unit includes a heated roller that heats the media to a predetermined temperature to begin media surface preparation. The media preparation station 18 removes debris and loose particulate matter from the web surface to be printed and the pre-heater roller 22 is heated to a temperature that transfers sufficient heat to the media material for optimal ink reception on the web surface as it passes the marking stations 26. Each of the marking stations 26A, 26B, 26C, and 26D in FIG. 3 includes two staggered full width printhead arrays, each of which has three or more printheads that eject ink onto the web surface. The different marking stations eject different colored inks onto the web to form a composite colored image. In one system, the marking stations eject cyan, magenta, yellow, and black ink for forming composite colored images. The surface of the web receiving ink does not encounter a roller until it contacts the leveling roller 34. Leveling roller 34 modifies the temperature of the web and reduces any temperature differences between inked and non-inked portions of the web. After the temperature leveling, the ink is heated by non-contact heater 44 before the printed web enters the spreader 38. The spreader 38 applies pressure to the ejected ink on the surface of the web to smooth the roughly semicircular ink drops on the surface of the web and to encourage ink fill with the different colors and present a more uniform image to a viewer. The web material is then wound around the rewinding unit 40 for movement to another system for further processing of the printed web.
This system 10 also includes two load cells, one of which is mounted at a position prior to pre-heater roller 22 and the other is mounted at a position near the turn roller 30. These load cells generate signals corresponding to the tension on the web proximate the position of the load cell. Each of the rollers 22, 30, and 34 has an encoder mounted near the surface of the roller. These encoders may be mechanical or electronic devices that measure the angular velocity of a roller monitored by the encoder, which generates a signal corresponding to the angular velocity of the roller. In a known manner, the signal corresponding to the angular velocity measured by an encoder is provided to the controller 60, which converts the angular velocity to a linear web velocity. The linear web velocity may also be adjusted by the controller 60 with reference to the tension measurement signals generated by the load cells. The controller 60 may be configured with I/O circuitry, memory, programmed instructions, and other electronic components to implement a double reflex printing system that generates the firing signals for the printheads in the marking stations 26. The term “controller” or “processor” as used in this document refers to a combination of electronic circuitry and software that generate electrical signals to control a portion or all of a process or system.
The controller 60 may implement either a single reflex or a double reflex registration system to time the delivery of firing signals to printheads in a print zone of a web printing system. “Double reflex registration system” refers to a system that uses the angular velocity signals corresponding to the rotation of two or more rollers to compute the web velocity at a printhead positioned between the rollers. A single reflex registration system refers to a system that uses the angular velocity signals corresponding to the rotation of only one roller to compute a linear web velocity that is used to predict web positions and timing in a print zone. A double reflex control system is described in U.S. Pat. No. 7,665,817, which is entitled “Double Reflex Printing” and which issued on Feb. 23, 2010 and is owned by the assignee of the present application. The disclosure of this patent is expressly incorporated herein by reference in its entirety.
The system 10 may also include an imaging device 68, such as an image-on-web array (IOWA) sensor, that generates image data corresponding to a portion of the web passing the imaging device. The imaging device 68 may be implemented with a plurality of imaging sensors that are arranged in a single or multiple row array that extends across at least a portion of the web to be printed. The imaging device directs light towards the moving web and the imaging sensors generate electrical signals having an intensity corresponding to the light reflected off the web. The intensity of the reflected light is dependent upon the amount of light absorbed by the ink on the surface, the light scattered by the web structure, and the light reflected by the ink and web surface. The imaging device 68 is communicatively coupled to the machine controller 60 to enable the image data generated by the imaging device 68 to be received and processed by the controller 60. This image data processing enables the controller to detect the presence and position of ink drops ejected onto the surface of the web at the imaging device 68.
As noted above, the controller 60 uses the tension measurements from the two load cells along with the angular velocity measurements from encoders to compute linear web velocities at the rollers 22, 30, and 34. These linear velocities enable the processor to determine when a web portion printed by one marking station, station 26A, for example, is opposite another marking station, stations 26B, for example, so the second marking station can be operated by the controller 60 with firing signals to eject ink of a different color onto the web in proper registration with the ink already placed on the web by a previous marking station. When the subsequent marking station is operated too soon or too late, the ejected ink lands on the web at positions that may produce visual noise in the image. This effect is known as misregistration. Accurate measurements, therefore, are important in registration of different colored images on the web to produce images with little or no visual noise.
Accurate angular velocity measurements simplify the process of determining the linear velocity of the web at a particular position and the timing of the firing signals correlated to the linear web velocity. In previously known image registration systems, a constant diameter is used for each roller that is monitored by an encoder to generate an angular velocity signal, which is used to compute a linear web velocity. Assuming that the diameter of a roller remains constant may lead to inaccuracies in web velocity calculations. The inaccuracy may be particularly troublesome in heated rollers. These rollers include a heating element that is mounted within the roller or proximate the roller to heat the roller to a temperature above the ambient temperature of the environment of the roller. The heated roller may be used for such purposes as preconditioning the web for printing or the like. When the roller is heated, the material forming the rotating cylinder of the roller expands. This expansion is particularly apparent in rollers having cylinders formed from metal, such as aluminum or stainless steel. The changes in the diameter of the roller cylinder may be significant enough to affect the accuracy of the velocity computed for the web and the timing of the firing signals for the printheads that eject ink as the web passes by the printheads.
Other factors also contribute to the accuracy of the timing of the firing signals. For example, one factor affecting the registration of images printed by different groups of printheads is web shrinkage. Web shrinkage is caused as the web is subjected to relatively high temperatures as the web moves along the relatively long path through the web printing system. The high temperatures drive moisture content from the web, which causes the web to shrink. If the physical dimensions of the web change after one group of printheads has formed an image in one color ink, but before another group of printheads has formed an image in another color of ink, then the registration of the two images is affected. The change may be sufficient to cause misregistration between ink patterns ejected by the different groups of printheads. The amount of shrinkage depends upon the heat to which the web is subjected, the speed of the web as it moves over heated components, the moisture content of the paper, and the type of paper. Additionally, the amount of water in the web alters the elasticity of the web and the computations for web velocities with those changes. Addressing the web changes and roller changes during operation of a web printing system to reduce their impact on image registration is a goal in web printing systems.